Understanding magnetic remanence acquisition through synthetic sediment deposition experiments

Laboratory redeposition experiments have been carried out for decades to understand how sedimentary rocks acquire their remanent magnetizations. These experiments have always used natural sediments (often prepared from disaggregated rocks) or synthetic sediments where magnetic minerals (often synthetic) have been dispersed in natural quartz-sand, mud or an artificial mixture. The typical approach to describe how sedimentary rocks acquire their remanent magnetizations considers spherical or platey magnetic particles falling through a stagnant water column. Viewed in this way, the particle is subject to balanced inertial, viscous and magnetic torques, and the magnetic particles attain perfect alignment with the ambient field. Eventually the particles encounter the sub-surface, leading to mechanical interaction. Laboratory experiments of vertically natural falling particles have shown that the net effect of a depositional remanent magnetization is to shallow the remanent inclination in the rock. Misalignment of declination is negligible. Experiments have also revealed a field dependence of the magnetization which is orders of magnitude lower than the saturation remanence, contrary to what predicted by simple depositional remanence magnetization (DRM) theory. "Natural" experiments performed, however, yield extremely heterogeneous results because of the heterogeneous nature (size, shape, magnetic properties, surface charge, etc.) of the particles. Such experiments therefore fail to answer basic questions that concern the fundamental processeses involved in DRM acquisition. We report on depositional experiments conducted using spherical glass beads of well-constrained size distribution and composition, and well-characterized magnetic properties given by magnetite and other iron impurities within the glass. Experiments are carried out in glass tubes of different diameters placed in controlled magnetic fields. Results confirm the inclination error in sediments and show that the acquisition of the DRM intensity grows rapidly below ~100 minutes, after which it slowly approaches saturation, in some cases without reaching it after 10 days of continuous deposition. This behavior parallels the volume of deposited sediment over time. Inclination, on the other hand, has a more linear trend. Sediments deposited in the smaller tubes acquire an inclination that is closer to the field inclination than for the larger tubes and also saturates earlier. In the larger tubes saturation of the inclination is not achieved after 10 days of continuous deposition. The Experiments also confirm the field dependence of the magnetization. Numerical simulations indicate that the experimentally determined DRM is lower than the saturation remanence and predict that for spherical particles, rolling of the smaller grains as they settle on the larger grains can produce a substancial shallowing of the inclination and lowering of the remanence. A model of the effect of particle rolling on remamence intensity and inclination is presented.